gpu: host1x: Add locking in channel allocation

Add locking around channel allocation to avoid race conditions.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230901111510.663401-1-cyndis@kapsi.fi